Photochromism is a reversible action of a compound which quickly changes its color when it is irradiated with light containing ultraviolet rays such as sunlight or light of a mercury lamp and resumes its initial color when it is no longer irradiated with light but is placed in a dark place, and has been applied in a variety of uses.
Photochromism has been applied in a field of, for example, spectacle lenses, too, and there have been obtained plastic lenses imparted with photochromic properties by adding a variety of photochromic compounds having the above-mentioned properties. As the photochromic compounds, there have been known fulgimide compounds, spirooxazine compounds and chromene compounds.
Plastic lenses having photochromic properties are obtained by:    (i) a method of having the surfaces of a lens without photochromic properties imbibed with a photochromic compound (imbibition method);    (ii) a method of dissolving a photochromic compound in a monomer which is, then, polymerized to directly obtain a photochromic lens (in mass method); and    (iii) a method of forming a coating having photochromic properties on the surfaces of the lens (coating method).
Photochromic properties of the photochromic plastic lenses produced by these methods are intimately related to the properties of the resin (or the starting monomer composition) that becomes a matrix of the photochromic compound, and a variety of studies have heretofore been conducted in an effort to improve the photochromic properties by utilizing the above properties. For example, it has been succeeded in obtaining photochromic lenses having relatively good photochromic properties such as a color density and a fading rate by employing such means as lowering the glass transition temperature (Tg) of a base lens material enabling the photochromic molecules to easily move even in the high molecules, or by using a particular alkylene glycol dimethacrylate of a long chain and a polyfunctional methacrylate having three or more radically polymerizing groups in combination as starting monomers of the matrix resin to expand free spaces in the high molecules enabling the photochromic molecules to easily move (see prior art 1 appearing below).
However, the photochromic plastic lenses involve a problem concerning the light resistance caused by the deterioration due to photo oxidation of the photochromic compound though the degree of difference may differ depending upon the method of production or the kind of the matrix resin that is used. According to the method taught in the prior art 1, for example, Tg of the base lens material is lowered to increase imbibition of the photochromic compound. Therefore, flexibility of the lens material becomes too high and, as a result, oxygen permeability becomes high and the photochromic compound is easily deteriorated by photo oxidation Therefore, if the photochromic lens obtained by this method is used for extended periods of time, the base lens material tends to be colored in yellow prior to developing color or the color density decreases. Deterioration of the photochromic compound can be prevented to a considerable degree by contriving the monomers and the photochromic material for obtaining a plastic lens that serves as a base material (see prior art 2 appearing below) still leaving, however, room for improvement. As for the photochromic lens produced by the above coating method, the coating film containing the photochromic compound has a thickness which is as small as several tens of microns. Therefore, the light resistance becomes further smaller than those of the photochromic lenses obtained by the imbibition method and the in mass method.
In order to prevent the photochromic lens from deteriorating and to increase its light resistance, a method has been proposed to form a coating containing an organic ultraviolet-absorbing agent on the surfaces of the photochromic lens (see prior art 3 appearing below.). Here, as the ultraviolet ray-absorbing agent, there can be concretely exemplified benzophenone-type and benzotriazole-type ultraviolet ray-absorbing agents.                Prior Art 1: U.S. Pat. No. 5,739,243        Prior Art 2: Leaflet of International Laid-Open No. 01/05854        Prior Art 3: U.S. Pat. No. 6,547,390        
However, when an ultraviolet ray-absorbing coating is formed on the surface of the photochromic lens by using an ultraviolet ray-absorbing agent that is concretely disclosed in the above prior art 3, the ultraviolet ray-absorbing agent absorbs even ultraviolet rays of wavelengths effective for exciting the photochromic compound arousing a problem of a decreased color density of the lens.
According to the above prior art 3, further, an organic ultraviolet ray-absorbing agent is added to a silicone coating agent (most generally used coating agent) which comprises an alkoxysilane and a silica gel, and the ultraviolet ray-absorbing coating is formed by using the above coating agent. When the above coating agent is used, however, the coating becomes cloudy due to the precipitation of an ultraviolet ray-absorbing agent at the time of curing, and the quality of the lens decreases. Even when the degree of cloudiness is small, too, the ultraviolet ray-absorbing agent bleeds out from the interior of the coating after the use for extended periods of time, and the effect gradually decreases for preventing the oxidation and deterioration of the photochromic compound and the light resistance is not improved to a sufficient degree. The problem of precipitation of the ultraviolet ray-absorbing agent at the time of curing can be improved if the ultraviolet ray-absorbing agent is added in a decreased amount. To obtain a lens without virtually any problem from the standpoint of optical properties, however, it becomes necessary to considerably decrease the amount of the ultraviolet ray-absorbing agent. After all, it becomes difficult to sufficiently suppress the oxidation and deterioration of the photochromic compound, and the light resistance is not improved to a satisfactory degree.